Population dynamics and production of five crustacean zooplankters in a subtropical reservoir during years of contrasting turbidity

Abstract

SUMMARY 1. The temporal dynamics and demography of Meta‐diaptomus meridianus (Van Douwe), Lovenula excellens Kiefer. Daphnia gibba Methuen, D. barbata Weltner and Moina brachiata Jurine were studied for 2 years in a small bay of Lake le Roux (Orange River, South Africa). Total zooplankton biomass and population density were 1.4–3 times higher during the less turbid conditions of 1982/83 (Secchi depth transparency around 35 cm) than they were at around 25 cm Secchi depth during 1981/82, when D. barbata was absent.2. On average, instantaneous birth rates, rates of population change and death rates varied only slightly between years. Birth and death rates were considerably higher above 15°C than below 15°C. These rates correlated with one another and with zooplankton abundance both inter‐ and intra‐specifically suggesting that competitive interactions were important in population regulation. Mortality rates varied more strongly and consistently in a density‐dependent direction than did birth rates. In addition to depressed fecundity, the inferred survival of young was poor and population growth low, possibly because food shortage caused high post‐natal mortality.3. Estimates of annual production derived from finite birth rate values varied consistently with annual differences in biomass, and amounted to between 6 and 10 g m−2 y−1 dry wt. Annual P/B values varied from around 20 for the daphnids to 55 for the copepods and 75 for Moina. Apart from the latter, whose annual P/B ratio virtually doubled from 45 to 75 following reductions in turbidity, annual differences in P/B ratio were slight.